4.5 Evaluation and Examples

The eval-expr must produce a sandbox evaluator via make-evaluator or make-module-evaluator with the sandbox-output and sandbox-error-output parameters set to 'string. If eval-expr is not provided, an evaluator is created using make-base-eval. See also make-eval-factory.

If the value of current-print in the sandbox is changed from its default value, or if print-as-expression in the sandbox is set to #f, then each evaluation result is formatted to a port by applying (current-print) to the value; the output port is set to a pipe that supports specials in the sense of write-special, and non-character values written to the port are used as content. Otherwise, when the default current-print is in place, result values are typeset using to-element/no-color.

Certain patterns in datum are treated specially:

• A datum of the form (code:line code-datum (code:comment comment-datum ...)) is treated as code-datum for evaluation.

• Other uses of code:comment and code:blank are stripped from each datum before evaluation.

• A datum of the form (eval:alts show-datum eval-datum) is treated as show-datum for typesetting and eval-datum for evaluation.

• A datum of the form (eval:check eval-datum expect-datum) is treated like eval-datum, but check-datum is also evaluated, and an error is raised if they are not equal?.

• A datum of the form (eval:result content-expr out-expr err-expr) involves no sandboxed evaluation; instead, the content result of content-expr is used as the typeset form of the result, out-expr is treated as output printed by the expression, and err-expr is error output printed by the expression. The out-expr and/or err-expr can be omitted, in which case they default to empty strings.

  Normally, eval:result is used in the second part of an eval:alts combination.

• A datum of the form (eval:results content-list-expr out-expr err-expr) is treated like an eval:result form, except that content-list-expr should produce a list of content for multiple results of evaluation. As with eval:result, out-expr and err-expr are optional.

As an example,

#lang scribble/manual

@(require racket/sandbox

scribble/eval)

@(define my-evaluator

(parameterize ([sandbox-output 'string]

[sandbox-error-output 'string])

(make-evaluator 'typed/racket/base)))

@interaction[#:eval my-evaluator

(: my-sqr (Real -> Real))

(define (my-sqr x)

(* x x))

(my-sqr 42)]

uses an evaluator whose language is typed/racket/base.

If pretty-print? is true, the sandbox’s printer is set to pretty-print-handler.